As electronic equipment has recently become smaller in size and higher in density, circuit boards having electronic components mounted thereon have shifted from being single-sided circuit boards to double-sided, multi-layer circuit boards. High-density circuit boards capable of integrating an increasing number of circuits and components have accordingly been developed.
FIGS. 5A to 5G are sectional views of a conventional circuit-forming board providing a circuit board for illustrating a method of manufacturing the board disclosed in Japanese Patent Laid-Open Publication No. 6-268345.
As shown in FIG. 5A, films 17 are attached onto both surfaces of pre-preg sheet 12 by a laminating method using, e.g. a thermal roll. Pre-preg sheet 12 is obtained by drying woven fabric of glass fiber (reinforcing material) impregnated with varnish of thermosetting resin, such as epoxy resin. Pre-preg sheet 12 is in a B-stage and has a thickness of 100 μm. Film 17 is made of polyethylene terephthalate (PET) having a thickness of 20 μm. Film 17 may be coated with thermosetting resin, such as epoxy resin.
As shown in FIG. 5B, via-holes 18 are formed in pre-preg sheet 12 by a process, such as a laser technology.
Next, as shown in FIG. 5C, via-holes 18 are filled with conductive paste 13 obtained by kneading thermosetting resin, hardening agent, solvent, and conductive particles, such as copper powder,
Then, as shown in FIG. 5D, films 17 are peeled off. Conductive paste 13 protrudes from pre-preg sheet 12. Then, copper foils 19 are placed on both surfaces of pre-preg sheet 12.
Then, copper foils 19 are pressed by a hot-pressing apparatus (not shown) so as to heat pre-preg sheet 12, copper foils 19, and conductive pastes 13. This operation hardens pre-preg sheet 12 thermally, and compresses conductive paste 13 as to allow conductive paste 13 to be electrically connected to copper foils 19, as shown in FIG. 5E. At this moment, the resin impregnated in pre-preg sheet 12 flows to outsides of pre-preg sheet 12, providing resin 12A.
Then, as shown in FIG. 5F, an unnecessary portion at an end is cut off, thus providing the circuit-forming board. Then, copper foils 19 are etched to have predetermined circuit patterns to provide circuit patterns 15 shown in FIG. 5G, thereby providing a double-sided circuit board. This board is cut to provide circuit board 16 having a predetermined size. A solder resist may be formed on circuit pattern 15 in order to prevent the pattern from having an unnecessary solder attached thereto. Circuit patterns 15 may be subjected to a finishing process, such as a plating process.
FIGS. 6A to 6E are sectional views of a multi-layer circuit board obtained by using circuit board 16 manufactured by the method shown in FIGS. 5A to 5G for illustrating a method of manufacturing the multi-layer circuit board. Pre-preg sheets 12B filled with conductive paste 13A and copper foils 19A are positioned and placed on both surfaces of circuit board 16 as a core, and are heated and pressed to provide multi-layer circuit board 20 having circuit patterns 15A on both surfaces thereof.
In the method of manufacturing the circuit board shown in FIGS. 5A to 5G, conductive paste 13 and 13A heated and pressed by, e.g. a hot-press apparatus may cause the following problem. FIGS. 7A to 7C are sectional views of the circuit-forming board shown in FIGS. 5A to 5G. As shown in FIG. 7A, copper foils 19 are placed on both surfaces of pre-preg sheet 12 having via-holes 18 filled with conductive paste 13. These are sandwiched between metal plates 14, heated, and pressing, being compressed. A pressure applied to them may cause conductive pastes 13 to deform, distorting the shapes of pastes 13, as shown in FIG. 7B
Then, as shown in FIG. 7C, circuit patterns 15 are formed, and conductive pastes 13 become interlayer-connecting portions 13D and 13C adjacent to each other. If the circuit board has a high density, the distance for insulation between interlayer connecting portions 13D and 13C is reduced due to the deformation of conductive paste 13, thereby reducing electrical insulation between the pastes.
When pre-preg sheet 12 is heated and pressed, the resin impregnated into pre-preg sheet 12 flows. When an excessive pressure is applied to conductive paste 13, conductive paste 13 spreads in in-plane direction 12D more than in thickness direction 12C. Thereby, the conductive particles in conductive pastes 13 are not pressed strongly and do not contact each other, so that sufficient electrical connection cannot be obtained. This phenomenon appears significantly if the diameter of each via-hole 18 is smaller than the thickness of pre-preg sheet 12.
As shown in FIGS. 6A to 6E, circuit board 16 as the core has a rough surface provided by circuit patterns 15 and has an uneven thickness, during the manufacturing of multi-layer circuit board 20. The rough surfaces and the uneven thickness of circuit board 16 cause conductive paste 13A on both surfaces to be compressed unstably by the heating and pressing, thereby causing conductive paste 13 to deform more easily.